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An Islamist commander was reported killed in one of those clashes, rights group says

Wounded Syrians streamed across the Turkish border seeking first aid Wednesday after a car bombing in a border town that's become a battleground in the fight between Syrian rebel factions.

The blast in the Syrian town of Jarablus shook windows about a kilometer away in the Turkish village of Karkamis, said Selami Yilmaz, a Karkamis resident.

"We don't even have enough ambulances to keep up with how many wounded are coming across," Yilmaz told CNN. He said he has lent his car to authorities to help get the wounded to hospitals.

The London-based Syrian Observatory for Human Rights said the car bomb exploded near a cultural center controlled by fighters from the Islamic State of Iraq and Syria. That al Qaeda-linked force has taken control of much of northern Syria amid that country's bloody civil war. The Syrian Observatory said heavy clashes were still going on in Jarablus.

ISIS, which has also taken control of parts of western Iraq, has attempted to impose strict Islamic law in towns where it holds sway. Rebel troops who had been battling forces loyal to Syrian President Bashar al-Assad launched an effort to push back the Islamists two weeks ago, but that offensive appears to have faltered after hundreds of deaths on both sides, observers report.

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Yilmaz said the Free Syrian Army, the leading Western-backed rebel force, has been losing ground in Jarablus. Dozens of its fighters have fled amid intense fighting, while wounded FSA troops have been pouring across the border into Turkey for treatment.

Human rights groups reported 183 deaths across Syria on Tuesday alone, all but 40 of them combatants on one side or the other. Heavy fighting between ISIS and rebel factions was under way around Aleppo, Syria's largest city, as well as the ISIS-held city of Raqqa and the border town of Saraqeb, where a Belgian man who had become the regional ISIS "emir" was among the dead, the Syrian Observatory reported.

The group also said ISIS executed two civilians from a Kurdish village last week after they left their homes to buy bread in a nearby city. Their bodies were found with their hands tied behind their backs "and marks from torture evident on their bodies," the London-based organization reported.

CNN cannot independently verify daily death tolls, but the United Nations has said more than 100,000 people have been killed in Syria since the revolt against al-Assad began in 2011.